1. Field of Invention
The present invention relates to a hydraulic swivel drive for a grab which has two gripping arms or shells movable like tongs, comprising two swivel shafts parallel to each other and rotatably mounted in a drive housing, whose shaft ends are connectable with the gripper arms or shells, wherein the swivel shafts each have two oppositely threaded screw engagement portions, with which two pistons movable in opposite directions by means of hydraulic pressure for driving the swivel shafts are in screw engagement.
2. Background and Related Art
A swivel drive for a clamshell grab according to the prior art is known for example from DE 201 07 206 U1, in which the swivel shafts each are part of a hydraulic motor which is enclosed by the shell carrier formed as closed housing. On the two swivel shafts, which swivel the gripper arms or gripper shells open and closed, pipe sections each are seated, which are longitudinally shiftably, but non-rotatably guided and are in screw engagement with the respective swivel shaft, so that a longitudinal displacement of the pipe sections leads to a rotation of the swivel shafts. The longitudinal displacement of the pipe sections is effected by means of a common center piece which connects the pipe sections and is formed as piston which is accommodated in the interior of the housing that forms the cylinder for the piston and can be charged with hydraulic pressure through corresponding pressure chambers.
As in this known swivel drive the piston/cylinder volume is increasing continuously with increasing distance of the swivel shafts and hardly manageable forces thereby act on the housing, DE 203 19 227 U1 proposes to not form the entire shaft drive piece as piston, but to provide plunger pistons in the shaft drive piece, which are movable relative to the shaft drive piece by hydraulic pressure in corresponding plunger piston chambers and thereby can shift the shaft drive piece to and fro in the desired way.
In hydraulic swivel drives of this type high axial forces occur at the bearings of the swivel shafts, which result from the fact that the linear movement of the shaft drive piece is converted into the desired swivel shaft rotation by a screw engagement. To be able to produce the necessary high swivel shaft moments, the shaft drive piece must be driven axially with correspondingly high forces, which leads to correspondingly high axial reaction forces in the bearings of the swivel shafts. To absorb these forces, the swivel shafts usually are supported by means of axial plain bearings. This provides for the typical compact construction of the swivel drive and the shell carrier, whose outside dimensions cannot by extended arbitrarily by the gripper shells to be connected. Such axial plain bearing of the swivel shafts, however, was found to be disadvantageous for the efficiency of the drive.
To reduce the bearing resistance of the swivel shaft bearing, DE 20 2004 013 158 U1 proposes hydrostatic axial bearings for the swivel shafts, which are fed by the hydraulic pressure by means of which the shaft drive piece is shifted. However, this solution in turn requires a corresponding constructional effort, and in addition special constructive measures must be taken, in order to supply the pressure pockets of the hydrostatic bearings with pressure to a sufficient extent.
For the solution of the above-mentioned bearing problems and to manage the axial forces, DE 20 2006 013 101 U1 proposes the use of two pistons moving in opposite directions, which each are in screw engagement with both swivel shafts and thereby utilize oppositely threaded screw engagement portions of the swivel shaft, so that the swivel shafts are rotated into the one or other direction by the pistons moving apart from each other and inversely by the pistons moving towards each other. By such pistons movable in opposite directions the axial bearing pressure can be minimized, as the driving forces and reaction forces of the pistons largely compensate each other. However, in the known swivel drive according to DE 20 2006 013 101 U1 relatively high transversal forces act on the housing in a direction transversely to the longitudinal shaft axes, which require a correspondingly massive and therefore heavy formation of the housing. Due to the large-surface pistons, the housing shell which defines the cylinder pressure chambers also has a correspondingly large surface area, which under the influence of hydraulic pressure leads to large forces on the housing. In addition to the transversal forces high axial forces also act on the lateral housing covers through which the swivel shafts extend. Regardless of the compensation of the bearing forces on the shaft bearings, the bearing caps still are strongly loaded by the hydraulic pressure even with pistons moving in opposite directions, so that these bearing caps and their attachment to the housing base body also must be formed correspondingly massive.
Therefore, it is the object underlying the present invention to create an improved swivel drive of the type mentioned above, which avoids the disadvantages of the prior art and develops the latter in an advantageous way. Preferably, a lighter construction of the drive housing should be achieved, without therefore having to make concessions as regards the driving torque of the swivel shafts and the gripping force of the grab.